How pearls achieve nanoscale precision
Coaxing order from unpredictable layers, mollusks do what humans can't


Date:
October 21, 2021
Source:
University of Michigan
Summary:
In research that could inform future high-performance nanomaterials,
a study has uncovered how mollusks build ultradurable structures
with a level of symmetry that outstrips everything else in the
natural world, with the exception of individual atoms.



FULL STORY ==========================================================================
In research that could inform future high-performance nanomaterials,
a University of Michigan-led team has uncovered for the first time how
mollusks build ultradurable structures with a level of symmetry that
outstrips everything else in the natural world, with the exception of individual atoms.


==========================================================================
"We humans, with all our access to technology, can't make something with
a nanoscale architecture as intricate as a pearl," said Robert Hovden,
U- M assistant professor of materials science and engineering and an
author on the paper. "So we can learn a lot by studying how pearls go
from disordered nothingness to this remarkably symmetrical structure."
The analysis was done in collaboration with researchers at the Australian National University, Lawrence Berkeley National Laboratory, Western
Norway University and Cornell University.

Published in the Proceedings of the National Academy of Sciences, the
study found that a pearl's symmetry becomes more and more precise as it
builds, answering centuries-old questions about how the disorder at its
center becomes a sort of perfection.

Layers of nacre, the iridescent and extremely durable organic-inorganic composite that also makes up the shells of oysters and other mollusks,
build on a shard of aragonite that surrounds an organic center. The
layers, which make up more than 90% of a pearl's volume, become
progressively thinner and more closely matched as they build outward
from the center.

Perhaps the most surprising finding is that mollusks maintain the symmetry
of their pearls by adjusting the thickness of each layer of nacre. If
one layer is thicker, the next tends to be thinner, and vice versa. The
pearl pictured in the study contains 2,615 finely matched layers of nacre, deposited over 548 days.



========================================================================== "These thin, smooth layers of nacre look a little like bed sheets, with
organic matter in between," Hovden said. "There's interaction between
each layer, and we hypothesize that that interaction is what enables the
system to correct as it goes along." The team also uncovered details
about how the interaction between layers works.

A mathematical analysis of the pearl's layers show that they follow
a phenomenon known as "1/f noise," where a series of events that seem
to be random are connected, with each new event influenced by the one
before it. 1/ f noise has been shown to govern a wide variety of natural
and human-made processes including seismic activity, economic markets, electricity, physics and even classical music.

"When you roll dice, for example, every roll is completely independent
and disconnected from every other roll. But 1/f noise is different
in that each event is linked," Hovden said. "We can't predict it,
but we can see a structure in the chaos. And within that structure are
complex mechanisms that enable a pearl's thousands of layers of nacre to coalesce toward order and precision." The team found that pearls lack
true long-range order -- the kind of carefully planned symmetry that
keeps the hundreds of layers in brick buildings consistent. Instead,
pearls exhibit medium-range order, maintaining symmetry for around 20
layers at a time. This is enough to maintain consistency and durability
over the thousands of layers that make up a pearl.

The team gathered their observations by studying Akoya "keshi" pearls,
produced by the Pinctada imbricata fucata oyster near the Eastern
shoreline of Australia. They selected these particular pearls, which
measure around 50 millimeters in diameter, because they form naturally, as opposed to bead- cultured pearls, which have an artificial center. Each
pearl was cut with a diamond wire saw into sections measuring three
to five millimeters in diameter, then polished and examined under an
electron microscope.

Hovden says the study's findings could help inform next-generation
materials with precisely layered nanoscale architecture.

"When we build something like a brick building, we can build in
periodicity through careful planning and measuring and templating," he
said. "Mollusks can achieve similar results on the nanoscale by using
a different strategy. So we have a lot to learn from them, and that
knowledge could help us make stronger, lighter materials in the future." ========================================================================== Story Source: Materials provided by University_of_Michigan. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. Jiseok Gim, Alden Koch, Laura M. Otter, Benjamin H. Savitzky,
Sveinung
Erland, Lara A. Estroff, Dorrit E. Jacob, Robert Hovden. The
mesoscale order of nacreous pearls. Proceedings of the
National Academy of Sciences, 2021; 118 (42): e2107477118 DOI:
10.1073/pnas.2107477118 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/10/211021175110.htm

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